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Heat pump cools but won't heat

I have a water-source heat pump (ClimateMaster Genesis) in a high-rise condo building that's been giving me trouble for years now. It periodically fails in heating mode. The first diagnosis was a bad thermostat, after which it heated normally for several months, only to fail again. Next diagnosis was a bad TXV. Brought in a second company for another estimate, and the second company said the TXV was fine, the unit just needed refrigerant. It then worked for about 6 weeks before failing again. Third diagnosis is that the coil is leaking refrigerant.

None of the theories make sense to me because, throughout all of this (over 2+ years), the heat pump works fine in cooling mode. In heating mode, the compressor cycles off after a minute or so, producing no hot air, while in cooling mode the compressor stays on and after being on for a while cools the air 10-15 degrees.

They added the refrigerant, then it worked for about 6 weeks before failing again. It now seems to be running fine in cooling mode (it's been in the 80's here already)... can the compressor run in cooling mode with low refrigerant, but not in heating mode with the same level of refrigerant?

If it is leaking refrigerant they have to fix the leak, because the reversing valve won't work if refrigerant gets too low. Sounds like it leaked down some in 6 weeks.

Which TXV did they change out the indoor or outdoor TXV?

This may not be the problem as it could just be the leak; but I will list it anyway.

The indoor check valve is supposed to be open in the heating mode letting the refrigerant bypass the TXV, when it is sticking closed part the time causing the refrigerant to go through both the indoor TXV & the outdoor TXV which would drop the pressure way down causing compressor to overheat & cycle off on limit.

I believe it is rather rare for a ball check valve to stick.
The check balls are located inside the heat pump TXVs.

Is the reversing valve working properly.
When charged properly & not heating, try switching from heat to cool a few times at start-up to see if it begins to heat.

If it had been too low on refrigerant it shouldn't cool properly either; but may be low enough so the reversing valve doesn't cycle properly.

It could be something else...a good tech will pinpoint the problem & fix it.

It stopped heating again after only 6 weeks from the time they filled it. They never changed the TXV because the second company thought it was OK.

As far as I can tell, there's only one TXV. It's a water-source heat pump and everything's contained in one unit indoors (not sure, but I think the water loops through a cooling tower). The reversing valve seemed logical to me too, but the troubleshooting guide in the manual says that's likely only if it's the opposite problem (heating not cooling).